The present invention relates to a suspension control system for use in a vehicle.
One example of conventional suspension control systems is disclosed in U.S. Pat. No. 5,533,597. A system according to the second embodiment shown in the patent publication includes a shock absorber of the variable damping coefficient type interposed between the body of a vehicle and an axle. The system further includes an actuator for adjusting the damping coefficient of the shock absorber. An acceleration sensor is attached to the vehicle body to detect a vertical acceleration acting on the vehicle body. An integrator circuit integrates the acceleration detected with the acceleration sensor to obtain the vertical velocity (absolute velocity, not relative velocity) of the vehicle body. Then, the absolute value of the vertical acceleration of the vehicle body is obtained, and the vertical velocity of the vehicle body obtained by the integration is divided by the absolute value of the vertical acceleration. The actuator is instructed to adjust the damping coefficient of the shock absorber on the basis of the value obtained by the division, thereby effecting vibration damping control for the vehicle body.
The above-described conventional suspension control system effects control resembling a control method based on the sky-hook damper theory.
According to the sky-hook damper theory, the damping coefficient C1 of the shock absorber (damper) provided between the vehicle body and the axle is obtained as follows.
Assuming that:
V: the vertical absolute velocity of the vehicle body (sprung mass);
X: the vertical absolute velocity of the axle (unsprung mass);
CZ: the damping coefficient of an imaginary shock absorber (damper) as provided between the vehicle body and a point in an absolute coordinate system;
if the following condition is satisfied;
V(Vxe2x88x92X) greater than 0
the damping coefficient C1 is determined as follows:
C1=CZV/(Vxe2x88x92X)xe2x80x83xe2x80x83(1)
If the following condition is satisfied;
V(Vxe2x88x92X) less than 0
the damping coefficient C1 is determined as follows:
C1=0xe2x80x83xe2x80x83(2)
In the above-described conventional suspension control system, a vertical acceleration acting on the sprung mass is detected with only the vertical acceleration sensor provided on the vehicle body without using a stroke sensor, and the damping coefficient C1 is determined on the basis of the detected vertical acceleration as stated below. More specifically, because the vertical acceleration signal changes in a manner similar to that of the actual relative velocity (Vxe2x88x92X), the vertical acceleration signal M is used as an estimated relative velocity according to the following control rules in place of the actual relative velocity (Vxe2x88x92X) between the sprung mass and the unsprung mass in the above Equation (1). That is, the conventional suspension control system obtains the damping coefficient C1 on the basis of the sky-hook damper theory as follows:
If V(Vxe2x88x92X) greater than 0,
C1=KV/Mxe2x80x83xe2x80x83(1a)
If V(Vxe2x88x92X) less than 0,
C1=Cminxe2x80x83xe2x80x83(2a)
In the above Equations (1a) and (2a), K is a constant and Cminxe2x89xa00.
With the acceleration sensor used in the above-described conventional suspension control system, the stroke of the shock absorber (damper) set out in FIG. 38 cannot be determined. Therefore, the suspension control system uses the above-described shock absorber of the variable damping coefficient type, in which when the damping coefficient for the extension stroke changes, the damping coefficient for the compression stroke becomes constant at a small value (Cmin), whereas when the compression-side damping coefficient changes, the extension-side damping coefficient becomes constant at a small value (Cmin).
Thus, the sign (positive or negative) of (Vxe2x88x92X) in FIG. 38 (i.e., the stroke of the shock absorber) is not judged, but instead when V greater than 0, a combination of C1 for extension and Cmin for compression is selected, and damping force for extension is controlled on the basis of C1. When V less than 0, a combination of Cmin for extension and C1 for compression is selected, and damping force for compression is controlled on the basis of C1.
The system may be arranged so that when C1 is positive, the damping coefficient for extension is controlled, whereas when C1 is negative, the damping coefficient for compression is controlled. In such a case, if it is possible to detect the vertical absolute velocity V of the vehicle body and the absolute value of the vertical acceleration signal M, it is possible to perform control approximate to the sky-hook damper theory by outputting C1 obtained by using the following Equation (1b):
C1=KV/|M|xe2x80x83xe2x80x83(1b)
Incidentally, the above-described prior art uses the vehicle body vertical acceleration signal M as data that can be regarded as approximation to the actual relative velocity (Vxe2x88x92X). In actuality, however, there is a phase difference between the vehicle body vertical acceleration signal 71 and the actual relative velocity 72, as shown in FIG. 39, under the influence of spring force and so forth [FIG. 39 shows an example of measurement of the vertical acceleration and relative velocity of the body of an automobile of a certain type when the vehicle body vibrates at 1 Hz, in which the phase of the vehicle body vertical acceleration signal 71 leads that of the actual relative velocity 72 by 131 degrees].
Because there is a phase difference between the vehicle body vertical acceleration signal 71 and the actual relative velocity 72, ideal damping characteristics such as those obtained on the basis of the sky-hook damper theory cannot be obtained with the conventional suspension control system that uses the vehicle body vertical acceleration signal 71 as an estimated relative velocity in-place of the relative velocity 72 corresponding to the relative velocity (Vxe2x88x92X) in the sky-hook damper theory. Accordingly, ride quality is not always good, particularly in a sprung resonance frequency band at relatively low frequencies (i.e. a frequency band in which vibration of the vehicle body influences ride quality to a considerable extent).
The present invention was made in view of the above-described circumstances.
Accordingly, an object of the present invention is to provide a suspension control system capable of obtaining damping characteristics closer to those obtained on the basis of the sky-hook damper theory by improving controllability in the sprung resonance frequency band, in particular, in consideration of the above-described phase difference due to spring force and so forth.
The present invention provides a suspension control system including a shock absorber having adjustable damping characteristics that is interposed between sprung and unsprung members of a vehicle. A sprung mass vibration detecting device detects vibration of the sprung member of the vehicle. A sprung mass absolute velocity detecting device obtains the absolute velocity of the vibration of the sprung member from the detected signal obtained from the sprung mass vibration detecting device. A relative velocity estimation unit adjusts the phase of the detected signal obtained from the sprung mass vibration detecting device to use the detected signal as an estimated relative velocity between the sprung and unsprung members. A control unit generates a control signal for controlling the damping characteristics of the shock absorber on the basis of the absolute velocity obtained from the sprung mass absolute velocity detecting device and the estimated relative velocity obtained from the relative velocity estimation unit and outputs the control signal to the shock absorber. The relative velocity estimation unit adjusts the phase of the detected signal so that the phase difference of the detected signal with respect to the actual relative velocity is minimized in the sprung mass resonance frequency band.
Preferably, the phase adjustment for the detected signal is made on the basis of adjusting parameters for the detected signal, and the characteristics of the adjusting parameters are changed according to the condition of the vehicle.
Preferably, the control device judges the road surface condition on the basis of the detected signal obtained from the sprung mass vibration detecting device, and the phase adjustment for the detected signal is made on the basis of adjusting parameters for the detected signal. Further, the characteristics of the adjusting parameters are changed according to the result of the judgment on the road surface condition.
Preferably, the control device converts the relative velocity obtained from the relative velocity estimation unit into a signal for generating the control signal on the basis of predetermined conversion characteristics and changes the conversion characteristics according to the condition of the vehicle or/and the road surface condition.
The above and other objects, features and advantages of the present invention will become more apparent from the following description of the preferred embodiments thereof, taken in conjunction with the accompanying drawings.